Considered a pipe dream not long ago, the 10-digit mark has become a realistic goal for some firms.

Cast your mind back 10 years and the idea of a $1 billion PR firm would seem laughable. Still emerging dazed and bruised from the dot-com era, the industry was regrouping in the wake of the bursting of a tech bubble many thought had changed business forever.

Today's market leader and 800-pound gorilla – Richard Edelman's eponymous firm – ...